A steam turbine as an embodiment of a turbomachine within the meaning of the present invention is to be understood as meaning any turbine or partial turbine through which a working medium in the form of steam flows. By contrast, gas and/or air flows through gas turbines as working medium, but this involves completely different temperature and pressure conditions than the steam in a steam turbine. Unlike gas turbines, in steam turbines by way of example the working medium at the highest temperature flowing to a partial turbine is at the same time also at the highest pressure. A steam turbine usually comprises a turbine shaft which is fitted with blades, is mounted rotatably and is arranged within an inner casing. As heated and pressurized steam flows through the interior of the flow space formed by the inner casing, the turbine shaft is made to rotate by the steam via the blades.
The blades of the turbine shaft are also known as rotor blades. Furthermore, it is customary for guide vanes to be mounted on the inner casing, engaging into the spaces between the rotor blades. The inner casing can also be referred to as the casing shell. A guide vane is usually held at a first position along an inner side of the steam turbine casing. It is usually part of a guide vane ring comprising a number of guide vanes which are arranged along an inner circumference of the inner casing. In this case, the air foil part of each guide vane faces radially inward.
Steam turbines or steam partial turbines can be divided into high-pressure partial turbines, intermediate-pressure partial turbines or low-pressure partial turbines. The entry temperatures and entry pressures of high-pressure partial turbines may be 600° C. and 300 bar.
There are known single-casing steam turbines which represent a combination of a high-pressure steam turbine and an intermediate-pressure steam turbine. These steam turbines are characterized by a common casing and a common turbine shaft and are also known as compact partial turbines.
Steam turbines for higher steam states usually use a material with a high chromium content. The material with a high chromium content is usually a chromium steel with a chromium content of 9 to 12% by weight. Hitherto, the same material used for the turbine shaft has also been used as material for the inner casing. This has been justified by the coefficient of thermal expansion needing to be identical for shaft and casing. The use of the material with a high chromium content for the turbine shaft and the inner casing leads to expensive designs of steam turbine.